Synthesis of novel AuPd nanoparticles decorated one-dimensional ZnO nanorod arrays with enhanced photoelectrochemical water splitting activity

Dual functionality of surface plasmon resonance and barrier layer on the photosensing and optical nonlinearity of ZnO nanorod arrays

The dual functionality of plasmonic light harvesting and barrier spacing between Au nanoparticles (NPs) and ZnO nanorod arrays (NRsA) are spotlighted to investigate their impact on the photoconversion and optical nonlinearity in the present study. The passivating Al₂O₃ barrier layer permits high-energy hot electron tunneling and injection from Au to the ZnO NRsA. The structural, vibrational, morphological/elemental, and optical properties of ZnO NRsA/a-Al₂O₃/Au were characterized by X-ray diffraction (XRD), Raman scattering, field emission scanning electron microscopy/energy dispersive X-ray spectroscopy (FE-SEM/EDX), and ultraviolet-visible-near IR (UV-Vis-near IR) absorption, respectively. The optoelectronic and nonlinear optical properties were analyzed by current-voltage measurement and z-scan tests under red laser (655 nm) irradiation, respectively. To highlight the effect of surface plasmon charge transport-based photosensing and photo-harvesting, the irradiated light source is selected to have a photon energy lower than the ZnO bandgap energy and detuning from LSPR. The transfer of photo-induced hot electrons from the Au NPs localized surface plasmon resonance (LSPR) to the ZnO NRsA translates into photocurrent generation in photosensing performance. The reverse saturable absorption process is changed to saturable absorption after intercalating the Al₂O₃ spacing layer into the ZnO NRsA/Au interface. The typical values of the nonlinear refraction index and absorption coefficient are calculated as n₂ = +2.38 × 10^-5 cm² W^-1 and β = -0.17 cm W^-1 for the sandwiched ZnO NRsA/Al₂O₃/Au heterostructure, respectively. The sandwiched ZnO NRsA/amorphous Al₂O₃/Au heterostructure exhibits strong nonresonant optical nonlinearity, which has an excellent figure of merit for optical switching.

One-pot, ligand-free, room-temperature synthesis of Au/Pd/ZnO nanoclusters with ultra-low noble metal loading and synergistically improved photocatalytic performances

Au/Pd/ZnO nanoclusters with ultra-low noble metal loadings were prepared by a one-step and ligand-free method at room temperature. HRTEM, ICP-MS, XPS, and elemental mapping analysis confirmed that the obtained Au/Pd/ZnO nanoclusters were composed of ZnO nanoclusters decorated with well-dispersed AuPd nanoparticles. Au/Pd/ZnO nanoclusters exhibited higher photocatalytic activity compared with those of pristine ZnO, Au/ZnO and Pd/ZnO. Moreover, the high catalytic activity of Au/Pd/ZnO nanoclusters could be maintained even after 5 cycles of photocatalytic reaction. A mechanism for the enhanced photocatalytic performance was also suggested, which was in light of the synergistic effects of the SPR effect from Au and the improved photogenerated charge carrier separation from Pd.

Localized surface plasmon-enhanced photoelectrochemical water oxidation by inorganic/organic nano-heterostructure comprising NDI-based D-A-D type small molecule

This research article reports the visible-light-driven photoelectrochemical water oxidation performances of the plasmonic Au-Pd nanoparticle-decorated inorganic/organic nano-heterostructures (NHs)-B-TiO₂/NDIEHTh@Au-Pd. The inorganic constituent of the NHs consists of boron-doped TiO₂ nanorods (NRs) grown on fluorine-doped tin oxide (FTO) coated glass substrate. The organic part (NDIEHTh) consists of an acceptor naphthalene diimide (NDI)-based donor-acceptor-donor (D-A-D) type small molecule, in which thiophene serves as the donor. Because of the benefits of the localized surface plasmon resonance (LSPR) effect, the Au-Pd binary alloy nanoparticles substantially ameliorate the visible-light-driven photoelectrochemical performances of the B-TiO₂/NDIEHTh@Au-Pd NHs photoanode compared to the B-TiO₂/NDIEHTh NHs photoanode. The photocurrent densities exhibited by the B-TiO₂/NDIEHTh NHs, and B-TiO₂/NDIEHTh@Au-Pd NHs photoanodes at 1 V vs Ag/AgCl are 0.68 mA/cm² and 1.59 mA/cm², respectively, manifesting 209% and 623% increments in the photocurrent density compared to that shown by B-TiO₂ NRs photoanode. Besides, the B-TiO₂/NDIEHTh@Au-Pd NHs photoanode offers a significantly cathodically shifted water oxidation potential, reduced charge transfer resistance, better surface injection efficiency, and most importantly, superior photostability compared to the B-TiO₂/NDIEHTh NHs photoanode. The enhancement in the different photoelectrochemical performances could be attributed to the various advantages of LSPR, such as enhanced light absorbance, light concentration, hot electron injection, and plasmon-induced resonance energy transfer.

Interface engineering of heterojunction photocatalysts based on 1D nanomaterials

1D nanomaterial-based heterojunctions with unique structures and outstanding physicochemical properties are divided into several types including type II heterojunction, p–n type heterojunction, Schottky junction, Z-type heterojunction, and S-scheme heterojunction.

Free carrier enhanced depletion in ZnO nanorods decorated with bimetallic AuPt nanoclusters

The decoration of semiconductor nanostructures with small metallic clusters usually leads to an improvement of their properties in sensing or catalysis. Bimetallic cluster decoration typically is claimed to be even more effective. Here, we report a detailed investigation of the effects of Au, Pt or AuPt nanocluster decoration of ZnO nanorods on charge transport, photoluminescence and UV sensitivity. ZnO nanorods were synthesized by chemical bath deposition while decoration with small nanoclusters (2-3 nm in size) was achieved by a laser-ablation based cluster beam deposition technology. The structural properties were investigated by scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, and the optoelectronic properties by current-voltage and photoluminescence measurements. The extent of band bending at the cluster-ZnO interface was quantitatively modeled through numerical simulations. The decoration of ZnO nanorods with monometallic Au or Pt nanoclusters causes a significant depletion of free electrons below the surface, leading to a reduction of UV photoluminescence, an increase of ZnO nanorod dark resistance (up to 200 times) and, as a consequence, an improved sensitivity (up to 6 times) to UV light. These effects are strongly enhanced (up to 450 and 10 times, respectively) when ZnO nanorods are decorated with bimetallic AuPt nanoclusters that substantially augment the depletion of free carriers likely due to a more efficient absorption of the gas molecules on the surface of the bimetallic AuPt nanoclusters than on that of their monometallic counterparts. The depletion of free carriers in cluster decorated ZnO nanorods is quantitatively investigated and modelled, allowing the application of these composite materials in UV sensing and light induced catalysis.

Enhancing the quasi-theoretical photocurrent density of ZnO nanorods via a lukewarm hydrothermal method

A ∼10-μm-long one-dimensional (1D) ZnO nanorod array (NRA) vertically oriented on a fluorine-doped tin oxide (FTO) coated glass substrate is successfully fabricated via a lukewarm hydrothermal method. The reflection of light from the rough surface of this ultralong ZnO NRA, resulting from the variation in the characteristic length of individual ZnO NRs in a tapered geometry, is largely suppressed. This in turn favors the ZnO NRA as a photoelectrode effectively harnessing UV-light for solar water splitting, as evidently manifested in the quasi-theoretical photocurrent density that reached ∼0.9 mA cm^-2 at 1V_Ag/AgCl. A further contribution to such an outstanding performance stems from additional photocurrent generation by the ZnO NRA upon visible light illumination. This is attributed to a variety of native defects and the surface hydroxyl groups present in the ZnO NRA, giving rise to the mid-gap states that mediate the associated electronic transitions. Moreover, those lattice imperfections further boost the carrier concentration of the ZnO NRA to facilitate the carrier transport which in turn enhances the photoelectrochemical activity.

InGaN Nanorods Decorated with Au Nanoparticles for Enhanced Water Splitting Based on Surface Plasmon Resonance Effects

Photoelectrochemical (PEC) water splitting has great application potential in converting solar energy into hydrogen energy. However, what stands in the way of the practical application of this technology is the low conversion efficiency, which can be promoted by optimizing the material structure and device design for surface functionalization. In this work, we deposited gold nanoparticles (Au NPs) with different loading densities on the surface of InGaN nanorod (NR) arrays through a chemical solvent route to obtain a composite PEC water splitting system. Enhanced photocatalytic activity, which can be demonstrated by the surface plasmon resonance (SPR) effect induced by Au NPs, occurred and was further confirmed to be associated with the different loading densities of Au NPs. These discoveries use solar water splitting as a platform and provide ideas for exploring the mechanism of SPR enhancement.

Current progress in developing metal oxide nanoarrays-based photoanodes for photoelectrochemical water splitting

Solar energy driven photoelectrochemical (PEC) water splitting is a clean and powerful approach for renewable hydrogen production. The design and construction of metal oxide based nanoarray photoanodes is one of the promising strategies to make the continuous breakthroughs in solar to hydrogen conversion efficiency of PEC cells owing to their owned several advantages including enhanced reactive surface at the electrode/electrolyte interface, improved light absorption capability, increased charge separation efficiency and direct electron transport pathways. In this Review, we first introduce the structure, work principle and their relevant efficiency calculations of a PEC cell. We then give a summary of the state-of the-art research in the preparation strategies and growth mechanism for the metal oxide based nanoarrays, and some details about the performances of metal oxide based nanoarray photoanodes for PEC water splitting. Finally, we discuss key aspects which should be addressed in continued work on realizing high-efficiency metal oxide based nanoarray photoanodes for PEC solar water splitting systems.

Exposing the photocorrosion mechanism and control strategies of a CuO photocathode

A CuO photocathode modified with TiO₂ and Pt displays superior photocorrosion stability in PEC water splitting.

Synthesis of AuPd nanoparticle-decorated graphene-coated ZnO nanorod arrays with enhanced photoelectrochemical performance and stability

AuPd nanoparticle-decorated graphene-coated ZnO nanorod (ZNR) array electrodes (ZNR@Gr/AuPd) were synthesized via electrostatic self-assembly followed by solution reduction methods.

Aerosol-Based Self-Assembly of a Ag-ZnO Hybrid Nanoparticle Cluster with Mechanistic Understanding for Enhanced Photocatalysis

A gas-phase-controlled synthetic approach is demonstrated to fabricate Ag-ZnO hybrid nanostructure as a high-performance catalyst for photodegradation of water pollutants. The degradation of rhodamine B (RhB) was used as representative, which were tested and evaluated with respect to the environmental pH and the presence of dodecyl sulfate corona on the surface of the catalyst. The results show that a raspberry-structure Ag-ZnO hybrid nanoparticle cluster was successfully synthesized via gas-phase evaporation-induced self-assembly. The photodegradation activity increased significantly (20×) by using the Ag-ZnO hybrid nanoparticle cluster as a catalyst. A surge of catalytic turnover frequency of ZnO nanoparticle cluster (>20×) was observed through the hybridization with silver nanoparticles. The dodecyl sulfate corona increased the photocatalytic activity of the Ag-ZnO hybrid nanoparticle cluster, especially at the acidic and neutral pH environments (maximum 6×), and the enhancement in catalytic activity was attributed to the improved colloidal stability of ZnO-based nanoparticle cluster under the interaction with RhB. Our work provides a generic route of facile synthesis of the Ag-ZnO hybrid nanoparticle cluster with a mechanistic understanding of the interface reaction for enhancing photocatalysis toward the degradation of water pollutants.

Morphological and optical properties of PdxAg1-x alloy nanoparticles

Alloy nanoparticles (NPs) can offer a wide range of opportunities for various applications due to their composition and structure dependent properties such as multifunctionality, electronic heterogeneity, site-specific response, and multiple plasmon resonance bands. In this work, the fabrication of self-assembled PdxAg1-x NPs alloy nanostructures with distinct size, density, shape, and composition is demonstrated via the solid-state dewetting of sputtered Pd/Ag thin films on c-plane sapphire. The initial stage of bilayer dewetting exhibits the nucleation of voids, followed by the expansion of voids and cluster breakdown and finally shape transformation along with the temperature control. Bilayer composition shows a substantial influence on the dewetting such that the overall dewetting is enhanced along with the increased Ag composition, i.e. Pd0.25Ag0.75 > Pd0.5Ag0.5 > Pd0.75Ag0.25. On the other hand, the size and density of NPs can be efficiently controlled by varying the initial thickness of bilayers. Reflectance peaks in UV and near-infrared (NIR) regions and a wide absorption band in the visible region arisen from the surface plasmon resonance are observed in reflectance spectra. The peak intensity depends on the composition of PdxAg1-x NPs and the NIR peaks gradually blue-shift with the size decrement.

Free-standing red phosphorous/silver sponge monolith as an efficient and easily recyclable macroscale photocatalyst for organic pollutant degradation under visible light irradiation

Traditional nano-sized photocatalysts in powdery form suffer from difficulties in recyclability, and have to be immobilized before practical applications. In this study, red phosphorous/silver (red P/Ag) sponge monolith was discovered to be a new free-standing macroscale photocatalyst, which was fabricated by a one-pot hydrothermal method without using any organic surfactants or templates for the first time. The elemental red P not only functioned as a reducing agent in the formation of Ag sponge monolith during the synthesis processes, but also as the active photocatalyst in-situ immobilized onto the Ag sponge. The as-prepared red P/Ag sponge monolith showed enhanced photocatalytic activity than that of traditional pure powdery red P by a factor of 3.1 times for organic pollutants (i.e. Rhodamine 6G, phenol) degradation under visible light irradiation. The enhancement was mainly attributed to the function of Ag sponge served as good electron sink to trap photo-generated electrons. More importantly, such free-standing macroscale photocatalyst can be easily recycled without activity deterioration. As a proof of concept, this work provides new insights into not only for the development of red P-based elemental photocatalysts, but also for the one-pot fabrication of novel free-standing macroscale materials with excellent photocatalytic activity for practical environmental applications.

Controllable design of natural gully-like TiO2–ZrO2 composites and their photocatalytic degradation and hydrogen production by water splitting

Gully-like TiO₂–ZrO₂ composites prepared using an instant centrifugation and one-step hydrolysis method exhibited good photocatalytic degradation and enhanced hydrogen evolution activity.